Imagine a perfectly reflecting box with a homogenous distribution of photons within it with total energy E (not entirely sure how close to possible this is, but don't worry about that). I would have thought this box's gravitational field would be very much like the combination of the the box's effect and a mass E/c^2 inside the box. Intuitively, it is quite easy to see the light provides a little extra inertia (because of the effect of motion of the box on the reflections of photons on the walls - the pressure of the photons on the walls is unbalanced if the box is accelerated. Hopefully the arithmetic means that this inertia is the same as there would be from a mass of E/c^2 regardless of the box's dimensions! In the Einstein field equations, if the box is in an inertial frame I would expect the light within the box to have no direct effect, because the net momentum flux is zero in any direction. But perhaps the reflection of the photons off the walls does all the work - with momentum being continually supplied to the photons from the box at every surface? Is this the non-zero contribution to the Einstein field equations?